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The National Zoo released the results of a necropsy performed on Mei Xiang's panda cub after its death Sept. 23. (Published Thursday, Oct. 11, 2012)

The National Zoo's panda cub died of a combination of lung and liver problems, zoo officials announced in a press conference Thursday morning.

The lungs of the six-day-old female cub were not fully formed, according to a necropsy. As a result, her liver didn't get enough oxygen, causing damage that led to her death Sept. 23. The cub may have been born prematurely, the zoo said.

The mortality rate for females born in captivity is 20 percent in the first year.

Zoo scientists are trying to learn more about how common liver and lung defects are in newborn pandas.

"As unfortunate as this was, this baby and studies of this baby post mortem are contributing to our knowledge of panda reproductive science," said Donald Moore, the zoo's associate director for animal sciences.

On Wednesday, keepers began removing the nest that Mei Xiang had made inside the panda house. Mei is no longer spending much time there, said News4's Megan McGrath.

As zoo officials prepared to start Thursday's news conference, Mei Xiang was let out into the yard, but kept close to the back fence while her keepers were inside the panda house.

After they left, she ventured further into the yard, and found a frozen fruit treat left there for her. But she spent much of her time with her back turned to visitors.

Mei Xiang is still down about 20 pounds from her usual 240, the zoo said. It is common for new panda mothers to not eat or drink for the first week of a cub's life, as they work to care for their cubs. Her food consumption is now up to around 80 to 85 percent of her normal intake of bamboo, fruits, vegetables and biscuits.

Shortly after the cub's death, keepers said she cradled a toy, a nurturing behavior that she had also shown before the cub's birth. Keepers described this as "expression of her natural mothering instinct."

Preliminary reports had indicated that the cub had fluid in its abdomen and some abnormalities in the liver, including discoloration. A day after the cub's death, Chief Veterinarian Suzan Murray said the free fluid in the abdomen was abnormal for a cub, and could be a symptom of liver problems.

Cubs are born tiny and helpless, at about the size of a stick of butter. The equivalent would be a full-term human baby fitting in the palm of your hand, a display at the panda habitat explains.

Cubs have been known to be accidentally crushed by their mothers, but that was not the case here. The zoo said early on that the cub had no external damage. She weighed slightly less than 100 grams, had nursed, and had a coat that was "just beautiful," Murray said Sept. 24.

The odds of Mei conceiving a cub after five consecutive pseudopregnancies since 2007 had been less than 10 percent, the zoo said shortly after the cub's birth. Keepers and panda fans were delighted by the cub's surprise arrival on Sept. 16.

Before the birth of the cub, zoo officials had said they were considering returning either Mei Xiang or the cub's father, Tian Tian, to China and swapping one for another panda with better breeding potential.

The future of the panda couple at the National Zoo will be decided later this fall, officials say. No decisions have been made yet.

Despite the cub's death, the pandas still drew many visitors Thursday.

Brian and Robin Ballard of Hopkins, Mich., followed news of the recent birth and came to see giant pandas for the first time Thursday.

"It's great to finally see them," said Brian Ballard, a teacher who used to work in a zoo. He remembers the first panda couple sent to Washington in 1972 as a gift to the U.S. after President Richard Nixon's historic visit to China.

"The cultural exchange that took place then, even by today's standards, I think is huge," Ballard said.